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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

SUGAR-DEPENDENT6 Encodes a Mitochondrial Flavin Adenine Dinucleotide-Dependent Glycerol-3-P Dehydrogenase, Which Is Required for Glycerol Catabolism and Postgerminative Seedling Growth in Arabidopsis^1,[C]

Warwick HRI, University of Warwick, Wellesbourne, Warwickshire CV35 9EF, United Kingdom

The aim of this study was to clone and characterize the SUGAR-DEPENDENT6 (SDP6) gene, which is essential for postgerminative growth in Arabidopsis (Arabidopsis thaliana). Mutant alleles of sdp6 were able to break down triacylglycerol following seed germination but failed to accumulate soluble sugars, suggesting that they had a defect in gluconeogenesis. Map-based cloning of SDP6 revealed that it encodes a mitochondrial flavin adenine dinucleotide (FAD)-dependent glycerol-3-P (G3P) dehydrogenase:ubiquinone oxidoreductase called FAD-GPDH. This gene has previously been proposed to play a role both in the break down of glycerol (derived from triacylglycerol) and in NAD⁺/NADH homeostasis. Germinated seeds of sdp6 were severely impaired in the metabolism of [U-¹⁴C]glycerol to CO₂ and accumulated high levels of G3P. These data suggest that SDP6 is essential for glycerol catabolism. The activity of the glycolytic enzyme phosphoglucose isomerase is competitively inhibited by G3P in vitro. We show that phosphoglucose isomerase is likely to be inhibited in vivo because there is a 6-fold reduction in the transfer of ¹⁴C-label into the opposing hexosyl moiety of sucrose when [U-¹⁴C]glucose or [U-¹⁴C]fructose is fed to sdp6 seedlings. A block in gluconeogenesis, at the level of hexose phosphate isomerization, would account for the arrested seedling growth phenotype of sdp6 and explain its rescue by sucrose and glucose but not by fructose. Measurements of NAD⁺ and NADH levels in sdp6 seedlings also suggest that NAD⁺/NADH homeostasis is altered, and this observation is consistent with the hypothesis that SDP6 participates in a mitochondrial G3P shuttle by cooperating with the cytosolic NAD-dependent GPDH protein GPDHC1.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Peter J. Eastmond (p.j.eastmond{at}warwick.ac.uk).

^[C] Some figures in this article are displayed in color online but in black and white in print.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.123703

^* Corresponding author; e-mail p.j.eastmond{at}warwick.ac.uk.

Received May 30, 2008; accepted June 23, 2008; published July 3, 2008.